The invention relates generally to the field of strain relief for cables and EMI shielding and more specifically to a bulkhead for use with SCSI and similar data cables that provides strain relief for the cables as well as EMI shielding.
Manufacturers of devices that use multiple SCSI (Small Computer Systems Interface) or similar data cables to connect to printed circuit boards mounted in computers or controllers face the problem of connecting the cables to a system that must also meet United States Federal Communications Commissions (FCC) ElectroMagnetic Interference shielding requirements. Such systems are often enclosed in cabinets that serve as EMI shields. If an opening is made in the cabinet to permit SCSI or similar cables to enter, this opening can allow the cable to become an "antenna" that leaks EMI emissions. Such leaks may cause interference with radio or television reception nearby, hence the regulations for EMI shielding.
In addition to EMI emission leakage, a second problem facing device manufacturers is that of strain relief for data cables. If several cables are simply attached to their respective expensive printed circuit boards through a sufficiently large hole in the cabinet, an accidental Full or stress on one or more of the cables might cause the cable(s) to damage the expensive printed circuit boards. When several devices are attached to boards in one cabinet, it is easy for a service technician to pull the wrong cable out, and--if too much force is used--rip the connector off the board, causing damage to the board. Since the printed circuit boards may cost thousands or tens of thousands of dollars, while the cables and cabinets are relatively inexpensive, it is desirable to minimize the possibility of any such damage.
One approach to solving the EMI shielding problem is to enclose the system within an EMI shield housing and then surround that housing with another outer cabinet or "skin." Typically, data cables then connect to connectors built into the EMI shield housing and through apertures in the outer housing. This is usually a much more expensive approach. It is harder to cool such a system. Such a design also tends to impose inherent limits on the number of SCSI or data cables that can be connected to the boards in the housing, and ultimately, to the number of boards in a particular cabinet. To the extent that multiple connectors are needed to connect one cable to one board in this type of design, signal strength can also be attenuated.
Another approach to solving the problem uses a cable EMI block made of two opposing metal plates with hemispherical channels formed on the inner surface of each plate so that one or more cables can be placed in the channels and held firmly in contact with the plates. The plates then function as grounding devices and also provide strain relief when they are clamped together.
A disadvantage of this approach is that it takes approximately forty-five minutes to an hour for a service technician to add or remove a full set of cables grounded by means of such an EMI block in a typical cabinet. Several screws holding it together need to be unscrewed. For reassembly, the cables must be placed within the channels and the plates fastened together again. It is awkward to hold the plates and cables in place while reassembling. It may require new or additional tools for assembly. Since it is also a bulky device, when it is installed in a cabinet it tends to make it difficult for the technician to reach inside the cabinet and to work with bulkheaded connectors located below and beneath it in the cabinet. Thus it is much simpler to remove the block for those processes, even though that may increase the risk of damage to the boards. Since the block has to be unclamped to remove or add a cable, all the cables may be left to hang freely for nearly an hour while the service technician inserts or removes cable. An accidental strain or pull that applies too much force to one of the dangling cables can cause damage to the board to which the cable is connected.
A third disadvantage of this approach is that the hemispherical channels of the block, although designed to fit tightly with the grounding shield mesh on the exterior of a typical SCSI cable, to provide strain relief as well as shielding may not, in fact, always meet the goal of strain relief. Some of the channels may not be machined to the specified tolerance and the cable may be too loose in the block--even after the block is clamped shut--to provide good strain relief. If the openings in the plates are too loose to establish good strain relief, this approach also does not allow for any adjustment to occur.
Another disadvantage of this and other approaches is that they may require special tools to be used for their installation and removal, thus increasing the costs of a solution.
It is an object of the present invention to provide a strain relief and EMI shielding device for bulkheaded SCSI or other data cables that is both easy to use and significantly reduces the time needed to install or remove cables.
It is another object of the present invention to provide adjustable strain relief for multiple data cables that also provides EMI shielding.
Still another object of the present invention is providing a strain relief device with EMI shielding that can also be retrofitted in many cases to existing cabinets.
Yet another object of the present invention is providing a strain relief device with EMI shielding that requires no new tools for field installation or maintenance.